Synchronous drive systems, such as timing belt-based systems, are widely used in motor vehicles, as well as in industrial applications. In motor vehicles, for example, timing belts or chains are used to drive the camshafts that open and close the engine intake and exhaust valves. Also other devices such as water pumps, fuel pumps etc. can be driven by the same belt or chain.
Internal combustion engines produce many types of mechanical vibrations during their operation, and these vibrations are usually transmitted through the timing belt or chain in the synchronous drive system. A particularly intense source of mechanical vibrations is given by the intake and exhaust valves and the camshafts that open and close those intake and exhaust valves. Opening and closing the intake and exhaust valves leads to a type of vibration known as torsional vibration. When the frequency of these vibrations is close to natural frequency of the drive, system resonance occurs. In resonance the torsional vibrations and the span tension fluctuations are at their maximum.
As flexible mechanical structures, timing belts and chains are particularly susceptible to the deleterious effects of mechanical vibrations. Mechanical vibrations transmitted through the timing belt or chain cause fluctuations in belt or chain tension, which can lead to increased wear and reduced belt or chain life. Vibrations may also cause timing errors, and result in undesirable amounts of noise.
Conventional techniques to attenuate the vibrations include increasing the tension on the belt or chain and installing camshaft dampers. Camshaft dampers connect a source of inertia to a camshaft sprocket by a vibration-absorbing rubber or silicone. However, increasing the belt or chain tension increases the noise level and reduces the useful life of the belt or chain. Installing camshaft dampers is also an undesirable solution, because of their cost and/or because of lack of space.
In DE-A-195 20 508 (Audi AG), there is disclosed a wrapped belt drive for an internal combustion engine, the timing belt being wrapped around two driven pulleys coupled to the camshaft of the engine, and one drive pulley coupled to the crankshaft of the engine. The objective of the invention is to counter the torsional vibrations which are found in such belt drives. It is proposed to provide an additional torsional vibration through which the critical resonance can be moved to a range where it can either be tolerated, or does not arise. It is proposed in the citation to produce torsional vibrations by an “out of round” pulley, which is shown as consisting of one of the camshaft pulleys. The out of round pulley which is shown has four protruding portions and four receding portions arranged regularly around the pulley. It is said that the variations in the pulley profile introduce torsionals to the timing belt at the incoming or outgoing spans of the driven pulleys, which are superimposed on the dynamics of the combustion engine, and thus shift or eliminate the critical resonance range. A figure is shown which is said to show a graph of torsional vibrations of the timing drive in degrees camshaft over the RPM of the crankshaft. The total amplitude is shown, and also the dominant vibration of the second order and the less relevant vibrations of the fourth order are shown. A single example of a magnitude of eccentricity of an out of round pulley is given, but no teaching is given as to how to select the magnitude of the eccentricity, and the angular alignment of the out of round rotor relative to the other rotors, for any given conditions of type of engine, type of drive belt, and type of load. As has been mentioned, the objective of the invention in the citation is to counter the torsional vibrations in the belt drive, and not to deal with the source of the vibrations.
In Japanese Utility Model JP 62-192077 (Patent Bulletin No. HEI 1-95538) of 1987 (Hatano et al/Mitsubishi), there is disclosed a tension equalising driving device which transmits the rotation of a drive pulley to a driven pulley by a belt drive such as a timing belt in an internal combustion engine. There is shown a timing belt arrangement in which a toothed pulley of the drive shaft of a camshaft is driven by an oval timing belt driving sprocket connected to the drive shaft of an internal combustion engine. The teaching of the citation is that the drive pulley is made oval in shape so as to give the drive belt a tension fluctuation with a phase opposite to that of the tension fluctuation in the belt produced by the rotation of the internal combustion engine. It is said that the drive pulley is installed in such a way that it gives the drive belt a tension fluctuation with a phase opposite to that of the tension fluctuation of the belt already present. The oval drive sprocket is said to be a tension equalising device, and is provided to equalise the tension in the drive belt. A figure is shown of a graph illustrating the tension caused by the valve train torque and the tension caused by the tension equalising device (the oval drive sprocket), the two tensions being shown of the same magnitude and opposite phase. There is no specific teaching given as to how to determine the magnitude of the eccentricity of the oval drive pulley, nor how to relate the angular position of the drive pulley to the camshaft pulley which is driven by the belt. In addition, as discussed in Japanese Application No. HEI 9-73581 (Patent Bulletin No. HEI 10-266868) of 1997 (Kubo/Mitsubishi), it was subsequently determined by the Applicant in JP 62-192077 (HEI 1-95538) that the use of an oval sprocket as a crank sprocket has a number of difficulties and problems and is thus not desirable.